The present invention relates to an optical signal amplifier and, more specifically, such an amplifier made by integrated optics technology.
Optical waveguide signal amplifiers are known comprising a glass optical waveguide fiber doped with an optically active material, for example a rare earth metal oxide, to develop an amplifier effect by stimulated emission These amplifiers include means for injecting into the amplifier fiber, at one of its ends, pump light emitted from a laser at the wavelength of absorption of the doped glass amplifier fiber, along with modulated signal light at a wavelength in the spectral band of emission of the doped glass amplifier. The amplified optical signal is extracted from the waveguide at the other end of the amplifier fiber.
Such an amplifier is described in E. Desurvire et al., "High-gain erbium-doped traveling-wave fiber amplifier", Optical Letters, vol. 12, no. 11, pages 388-390, Nov., 1987. M. C. Farries et al., "Operation of Erbium-Doped Fiber Amplifiers and Lasers Pumped with Frequency-Doubled Nd:YAG Lasers", Journal of Lightwave Technology, vol. 7, no. 10, pages 1474-1477, Oct., 1989, also is directed to an amplifier of this type.
Such amplifiers are intended for use in long distance optical fiber telecommunications systems which today appear very promising. Present day systems include opto-electronic repeaters spaced out along the length of the fiber to amplify the optical signals, as the attenuation of the fiber is on the order of tenths of decibels per kilometer. These opto-electronic repeaters include an optical to electrical converter at the input, an electronic amplifier of the electrical signal, and an electrical to optical converter at the output. Accordingly, these repeaters are complicated, cumbersome and expensive.
Therefore it is desirable to replace these opto-electronic repeaters with optical signal amplifiers of the type described above, which present the advantage of eliminating the opto-electronic conversion of signals by acting directly on the optical signal. In order to achieve the level of amplification necessary, the length of the amplifier waveguide is currently of the order of one to several meters, for a signal transmitted at a wavelength of 1.5 .mu.m, which is one of the typical wavelengths used in optical telecommunications. In addition, the following additional components are also required: means for supporting the optical fiber, which is sensitive to perturbations; means for connecting the ends of the fiber to input and output fibers; and, means for optically coupling the amplifier fiber to a pump-laser. Assembly of all of these individual means is difficult, hence expensive, and the amplifier which results is cumbersome.
Laser waveguides have been fabricated using ion exchange in glass. In Najafi et al., "Ion-exchanged rare-earth doped waveguides", SPIE Vol. 1128 Glasses for Optoelectronics, 1989, pp. 142-144, an example is given of a slab waveguide made by Ag.sup.+ -Li.sup.+ exchange in a neodymium-doped lithium-silicate glass substrate. A dye laser operating at 590 nm was end-fire coupled to the waveguide and the output was measured to show fluorescence at the emission wavelength of the neodymium-doped glass. FIG. 3 of Najafi et al. depicts a slab waveguide with gratings serving as mirrors to create a laser oscillator.
An laser oscillator is not a signal amplifier, because the output power is not a function of the input signal. A laser oscillator requires both a gain medium and a feedback means. It is possible to create a laser oscillator with a short path length and very small gain in a single traverse of the gain medium (single pass gain). Multiple passes through the gain medium are necessary to develop the required power level. The single pass gain in such a laser oscillator is not sufficient for a practical signal amplifier.
Accordingly, means other than oscillation between mirrors are required to increase the gain of a signal amplifier. One means for increasing the single pass gain is increasing the amount of the optically active species in a signal amplifier path of a given length. However, there is a limit to the concentration of rare earth dopant that may be included in a glass host without severe deleterious effects. And, the total amount of dopant required in a system for operation at a particular wavelength absorption band is a function of the pump absorption efficiency (stimulated emission cross section) of the dopant at that wavelength. Therefore, for some dopants used in systems operating at particular absorption on wavelengths, a relatively long length of amplifier waveguide is required to achieve a practicable level of amplifier gain.
German patent publication DE OS 2 260 987, assigned to Siemens is directed to a laser oscillator wherein the active material is provided in the form of a spiral waveguide (2) sandwiched between a substrate (1) and a transparent dielectric (4). An electroluminescent material (5) covers the transparent dielectric and is used to excite light in the waveguide which oscillates back and forth between its two ends. This laser oscillator configuration is unworkable as a signal amplifier because of the oscillation and because it is not possible to introduce the signal into the spiral waveguide, only the pump light.
German patent publication 2 140 500, also assigned to Siemens is directed to a optical detector including a meandering crystalline neodymium waveguide which is glued to a substrate. The waveguide operates as a gated preamplifier for the detector, and the active material is stimulated from above by a stimulating light source through a transparent layer applied over the waveguide. The stimulating radiation is applied over the entire length of the waveguide and not at one end. The waveguide is not integrated in a glass body, but is single crystal structure that is glued to a substrate.
It is therefore an object of the present invention to provide an optical signal amplifier which is economical and not cumbersome and which requires assembly of a minimum number of components.
Another object of the present invention is a method for making such an amplifier which allows integration of the amplifier components which are necessary for connecting the amplifier waveguide to the optical fibers which transport the optical signal.